(1) Field of the Invention
The present invention generally relates to an intake device for an internal combustion engine mounted on motorcycles and various vehicles. More particularly, the present invention relates to a specific type intake device provided with a variable intake pipe length mechanism and an intake vacuum adjusting means.
(2) Description of the Prior Art
It is well known to one skilled in the art that an internal combustion engine is provided with a variable intake pipe length mechanism to vary the intake pipe length. Such mechanism has been designed so as to adjust the intake pipe length to be long during the low revolution speed of the engine or to be short during the high speed. In addition to this mechanism the engine has been combined with a variable fuel control type carburetor as taught by Japanese Patent Application Open-Publication No. Sho. 61-49124.
In order to facilitate a better understanding of the prior art, the effects and functions of the variable pipe length mechanism in compensating the air/fuel ratio will be explained in conjunction with FIGS. 18 to 21.
FIG. 18 shows two performance curves representing the relation between the engine speed and the power generated by the engine, one is where the engine is provided with a combination of a long intake pipe and an optimum carburetor. A (combination A), and the other is where the engine is provided with a combination of a short intake pipe and an optimum carburetor B (combination B). These curves cross at a specific engine speed Nex. Assuming that this cross point, Nex, is used as a boundary point, an ideal performance will be achieved if the engine with the combination A is operated for a low engine speed zone from zero to the boundary point Nex and the engine is then B operated with the combination for a high engine speed zone above than the boundary point Nex.
FIG. 19 shows the relation between the engine speed and the required flow quantity for the combination A and Fig. 20 shows the relation between the engine speed and the required flow quantity for the combination B. In these figures, the straight line I represents an air/fuel ratio required to operate an internal combustion engine ideally regardless of the intake pipe length.
When one carburetor, for example the carburetor B, is used for executing its combustion control, the performance curve C shown in FIG. 21 is plotted. This curve C is identified with the line I in the high speed range above the boundary point Nex and while the curve C is shifted to the poor side of the air fuel ratio in the low speed range. This fact means that the engine provided with only the variable intake pipe length mechanism cannot always apply to every driving condition. In order to compensate this fact the engine is further provided with a variable fuel adjusting type carburetor so as to enrich the air/fuel ratio.
However, the variable fuel adjusting type carburetor requires a complicated structure and causes a time delay for generating the compensating effect after switching the fuel control device such as a main jet to adjust its air/fuel ratio.
As the intake pipe length is increased, the inertia effect is also enlarged and remarkable pulsating wave of air flow is generated in the intake pipe. FIG. 22 shows the influence of the pulsating wave applied to a main nozzle N of the carburetor. As an intake valve V makes an intake port IN alternatively open or close, the pulsating wave is alternatively applied in the positive or negative direction to the main nozzle N. The total area of the negative pulsating wave is represented by .SIGMA.M and the total area of the positive pulsating wave is represented by .SIGMA.P. The fuel can be smoothly sucked out of the main nozzle N as long as the difference between these areas is kept within the following condition. EQU .SIGMA.P-.SIGMA.M&lt;0 (.SIGMA.P and .SIGMA.M are absolute values)
However, as the difference approaches zero the sucked fuel quantity is decreased, so that the air/fuel ratio also fluctuates.